How to assign istringstream and ifstream to an istream variable?

Question

I want to have a variable of type istream which can hold either the contents of a file or a string. The idea is that if no file was specified, the variable of type istream would be assigned with a string.

std::ifstream file(this->_path)

and

std::istringstream iss(stringSomething);

to

std::istream is

I've tried just assigning them to the istream variable like I would with other objects that inherit from the same base class, but that didn't work.

How to assign istringstream and ifstream to an istream variable?

Answer 1

Base class pointers can point to derived class data. std::istringstream and std::ifstream both derived from std::istream, so we can do:

//Note that std::unique_ptr is better that raw pointers
std::unique_ptr<std::istream> stream;

//stream holds a file stream
stream = std::make_unique<std::ifstream>(std::ifstream{ this->_path });

//stream holds a string
stream = std::make_unique<std::istringstream>(std::istringstream{});

Now you just have to extract the content using

std::string s;
(*stream) >> s;